Chapter 1
INTRODUCTION TO NORMAL BINOCULAR VISION

1.1 The end product of binocular vision

Normal binocular vision is defined as the integration of monocular sensory and motor visual information into a combined percept of the surrounding physical space. This visual percept is heavily edited by the brain. It is affected by visual memory, and we sometimes react to visual stimuli before they pass into consciousness. This book sets out the processes and equipment involved in that editing.

Human binocular vision has several advantages over monocular vision. The obvious advantage is single vision rather than double vision, or vision alternating between each eye. Next, the subtle difference between the right and left viewpoints allows the most accurate form of depth perception, stereopsis. It is possible to see the effect of the different viewpoints in binocular vision by holding a hand edge in front of the eyes, and then closing each eye in turn. Stereopsis assists primates in hand-eye coordination and in precise interception of mobile food sources. Stereopsis helps to identify threats - adversaries may be spotted moving across the visual field with monocular vision, but when stationary, three-dimensional vision helps to identify a specific threat from background visual information. This is known as figure-ground separation, or breaking camouflage. With binocular vision, the amount of binocular convergence used to fixate a target with each eye allows an approximate assessment of the target distance by triangulation. Binocular vision also helps with spatial localisation: visual attention can be concentrated on objects situated in the plane of the binocular fixation point, allowing distracting stimuli nearer or farther away to be ignored. Binocular perception has advantages over monocular vision in assessing surface curvature. It also allows enhanced surface material perception using lustre perception.

At a higher level of visual performance, fine stereopsis allows very precise detailed tasks, e.g. using binocular operating microscopes, or mapping the apparent height of terrain using stereoscopic photographs. The brain also averages the visual input when combining right and left eye images, so that an individual with early cataract, who sees a letter 'O' as a 'Q' with one eye and as an inverted 'Q' with the other eye, correctly perceives 'O' in binocular vision. This process, binocular visual summation, improves binocular performance over monocular for:

Figure 1.1 The human visual fields. With both eyes open and fixation on the central cross, binocular vision is possible where the right and left fields overlap. The lower triangle represents the highly variable influence of the nose. The grey areas show the monocular extensions of the binocular visual field on each side.

• high-contrast visual acuity, and the upper spatial frequencies of contrast sensitivity;
• absolute light detection at threshold of perception;
• threshold contrast sensitivity function;
• reaction time to flashing visual stimuli, e.g. sine-wave bar gratings. This can be important in several occupational situations.

Two eyes and binocular vision supply a paired and therefore spare organ, true for many of the body's functions as insurance against injury and disease. Two eyes also give a wider field of vision. In animals that are subject to predators, the horizontal visual field may extend to 360 degrees, i.e. panoramic vision. In humans, the horizontal binocular visual field is 120 degrees, with a further monocular field of about 45 degrees (the temporal crescents) on each side of the binocular field, on the horizontal (medial-lateral) axis passing through the eyes, but reducing to zero superiorly and inferiorly (Fig. 1.1). The nose reduces binocular field inferiorly. In animals the monocular and binocular visual fields vary according to the species (Fig. 1.2).

1.2 The requirements for binocular vision

The requirements for binocular vision are as follows.

• Two eyes, and a separation between the eyes called the interocular distance, generally about 65 mm in adult humans.
• A neural pathway to transfer the two images to the brain (Fig. 1.3).
• Neural processing systems to integrate the different types of raw visual information, such as luminosity, size, movement relative to the eye, colour and contrast. These systems also analyse and produce further percepts, such as distance, shape, movement relative to the body and stereopsis.
• Extra-ocular muscles to allow the object fixated to be imaged on appropriate retinal areas of each eye (Fig. 1.4).
• Motor control systems to govern voluntary and reflex eye movements - e.g. to maintain or vary fixation. Also there has to be a method of correlating binocular sensory input and binocular motor function: motor correspondence.
• Further enhancement of binocular perception is obtained by the triangulation of objects observed using head and body movements and the addition of other, monocular, clues to the total visual perception.

Figure 1.2 The visual fields of the pigeon, showing a 24-degree binocular field, and a total field of 340 degrees, mainly monocular (after Walls, 1942).

Figure 1.3 The human neural pathway for vision, from the retina to the visual cortex. LGN, lateral geniculate nucleus.

Figure 1.4 The extra-ocular muscles and the right orbit in outline. The orbit is represented as a cone extending backwards to the optic foramen. Only the superior rectus and the medial rectus are shown, with the two oblique muscles. While the recti pull backwards towards their origins at the rear of the orbit, the obliques pull towards the medial wall of the orbit. For clarity, the inferior and lateral rectus muscles are indicated but not labelled.

1.3 Monocular visual direction

Spatial sense is the body's recognition of the location of external objects, involving the tactile sense, hearing and vision. The determination of external locations by visual means involves the relationship between the external location, the eyes and the head position. Visual direction describes the visual position of an object in a two-dimensional plane, i.e. its vertical and horizontal location. To move from physical space, which exists without our presence, to visual space (the visual representation of physical space) involves initially the use of visual direction to build up the perceived picture. Complications in building up visual space may be illustrated by an experiment with inverted vision. It is possible to adapt to inverted vision (which can be produced by an optical system) so that after 2 weeks' wear, vision becomes upright. Upon removing the lenses it takes about 30 minutes of alternating upright and inverted vision before normal vision is stable.

The recognition of monocular visual direction is attained by the association of a visual receptor in the retina with the external position of an object imaged on that visual receptor. The line passing through the centre of the entrance pupil, to any object of regard is called a line of sight (Alpern, 1969). For an object fixated by the fovea, this line is known as the primary line of sight or, in clinical practice, the visual axis (Ogle, 1950). The entrance pupil is the image of the actual pupil formed by the cornea, as seen by an observer.

The visual axis is more strictly defined as the external light ray that, after refraction by the optical system of the eye, will fall on the fovea (Freedman and Brown, 2008). The fovea is the retinal area that receives images from objects observed straight ahead. Visual acuity and colour perception are normally best at the fovea. When the object of regard is imaged on the fovea, the oculomotor system ceases to initiate any eye movement. The fovea is thus the retino-motor zero point, or retino-motor centre.

Disambiguation note: the term 'zero point' is also used in relation to retinal correspondence (see section 4.5 in Chapter 4).

Note:

• In 1907 Maddox used the terms 'visual line' and 'fixation line' (Maddox, 1907).
• The visual axis must pass through the nodal point(s), and as there are two nodal points in the eye situated 7.13 and 7.41 mm behind the corneal vertex, a single visual axis cannot strictly connect the fovea with the object of regard (Rabbetts, 2007; Harris, 2010). For simplicity hereafter the terms 'visual axis' and 'primary line of sight' are used synonymously, and this is indicated in the text.
• The visual axis is not (usually) the same as the optic axis of the eye, which is why the anterior corneal reflection is not usually in the centre of the pupil (Fig. 1.5). The measurement of these axes is discussed by Dunne et al., (2005).

The primary line of sight (the visual axis) is said to have the principal visual direction, i.e. from the fovea to the object imaged on the fovea. All non-foveal retinal receptors have secondary visual directions. The angular value of a secondary visual direction is calibrated by reference to the primary visual direction. The general term 'line of sight' includes both primary...